Self Applying Brakes - Ghost or Gremlins?

Sounds like it could be a warped rotor(s). When you stop, after it is doing this, check each wheel with your hand. Pull the hubcaps if necessary, and touch the center of the rim area by the hub.

Are one or more of them extremely hot to the touch?

A warped rotor will rub against the pads. It's barely noticeable at first, but as you drive, it gets hotter. Heat causes expansion of the brake fluid, which increases the pad pressure against the rotor. The further you drive, the hotter, (and more pronounced), it will get.

All correct except for the last line of your post, as all master cylinders that I have looked at have to have a breather hole, usually in the filler cap. Not having one would tend to "suck" the fluid back after applying the brakes, the complete opposite of the problem being experienced....Being able to screw the cap down with a pressure already in it is very unlikely/as good as impossible.

The OP's problem is an age old one with some master cylinders (I did all my own car work for many years, it was a well known/documented problem even then that any good mechanic would/should learn in school!), therefore it demonstrates an almost complete lack of understanding of car brake problems by his present mechanics. I would go as far as to say that it is one of the "classic" problems of car brakes. Go somewhere else for he final fix.

Take the bad mechanic to court or at least get a 40% discount back on the parts and work done, you have a cheap upgrade and he is not completely out of brains pocket.

(I am sure that there are other possible scenarios for this problem, but far far more seldom than as you say the master cylinder not allowing fluid expansion back into the reservoir when not braking).

May I suggest that the OP looks carefully at the pedal and checks if any of the following are true, tip number 3 is my personal favourite:-

1) Carpet or a badly routed electrical cable are blocking the pedal from returning fully

2) Gunge is preventing the piston returning fully. This needs a full clean of the hydraulics system if true.

3) Does a spring "help" the pedal to return after braking, is the spring weak or damaged? An elastic cargo strap can be used to get the car to a better mechanic.

Simply pulling up on the brake pedal by hand may assist him in deciding what is wrong.

If then the car can then be driven with no problem........you know whats up....

By the way, the problem gets worse while driving as the fluid warms up and expands, heated by the extra rubbing of the pads on the rotors.....its simply got nowhere else to go....

Best of luck.

Just took it for a drive since the roads are relatively empty (small town/almost 9pm) and waited until it did it again. It took about 10 minutes of city driving and this time I WAS applying the brakes often and testing it in neutral to ensure that the wheels were still spinning freely. Each time I used the brakes I would then pull up HARD from underneath with the top of my foot as soon as I released the pedal. There was no play and no room to pull up. I was really hoping that would be it. When it began it didn't take long until it really started holding forcefully. As soon as I got in the garage I jumped out of the van and grabbed my IR thermometer. 648 on fronts and 354 on rear rotors were the high temps. Of course I wasn't applying pressure to the brake pedal anymore on the way home; in fact there were a few small hills I almost couldn't get over. I will find a shop tomorrow to test the master cylinder, and booster if they are willing to do it.

Nicely done.

No play is bad.

I am not the first to post that.

You have found the problem, master cylinder, or master cylinder/booster.

Deal with it!!

Right Andy, The other day I found that an "after market" kit was fitted to a Toyota....they were looking at all the wrong places, the master cylinder piston [rear one] was 1.5mm too long so that it did not allow fluid return to the reservoir.

Therefore always check new parts. Blow into mastercylinder outlet so see if the small return port is open with pistons in rest position. Front wheel bearings [on most vehicles] should have a wee bit of play so that the brake rotor can push pads away a little bit and to allow ghrease moving into bearings.

Thank you all very much. I am not sure if I am using the correct terminology. When I say "no play" I am referring to the pedal has no room to be raised any more when It was suggested that I pull up on it or use a spring/bungee cord to be sure it was returning to its full up position. There is no play when trying pull it up. When pushing down on the pedal it feels pretty normal. I couldn't get the local brake guys to look at it yesterday or today but Monday 7:30AM it will have a home for the day. Thanks, thanks thanks.

Exactly!

If you can not pull the pedal up any more, how do you know that the pedal is not pushing the internal piston of the master cylinder in by .001", .01", .10, or 1.0"? Without any free play, you can not know this.

Another way to test for this is to loosen the master cylinder from the booster by one turn on each of the nuts securing the two together. Good luck.

There needs to be a tiny bit of play in all the master cylinder/pedals that I have worked on over the years......you have the well known problem that occurrs when there is no play......you have no play......so I am guessing but I think you may have found the problem.....

I cannot promise that ALL cars in the world need play, but there is a good chance.

You need to see the place where the master cylinder piston is contacted by the pedal and there should be a tiny gap when the piston is fully returned....no contact. as there are probably several different ways to achieve this you need to apply what I have said to your vehicle.

You are very close!!!

"There was no play and no room to pull up."

That is exactly the symptom that you should expect to find if the master cylinder push rod is not allowing the cup in the master to return to the correct position. What you found is wrong for a car. Go to another car and try to pull up the pedal. There must be play in the pedal. There should be a rubber bumper to stop the return travel of the brake pedal when released. As a simple test, temporally remove this bumper, and retest. Cheaper than any other option now. Good luck.

Sorry I slighted you on the master cylinder return issue. You indeed did point it out first. To make up for it, I will vote you a few GAs on the next off topic posts you send.

LOL!!

You did check to make sure the parking brake is off? You obviously have pad dragging for whatever reason, find which one as suggested, be careful after driving it will be very hot! or if you can use a lift, just spin each wheel while the vehicle is elevated to see that each wheel spins freely, or not...The most common cause would be a caliper sticking, then maybe emergency/parking brake on or adjusted to tight....good mechanics are hard to find, ask around for a good one....

Thanks for all the comments. I thought I should follow up a bit.

Sorry I didn't list everything but I was beginning to feel like I was rambling on.

The vehicle does indeed stop in a straight line. All four calipers are engaging and not just a little bit. If one were to let off the gas at freeway speeds it would drop to 40mph almost instantly. The van will stop itself, and quite rapidly. Rotors have never been red hot but do vaporize water instantly. It is worth noting that I am deliberately not applying the brake pedal unless absolutely necessary and when it happens on the freeway I have not touched them at all.

It isn't the rotors or the parking brake. I checked that since it has rotor/drum combination on the rear wheels. While investigating that was when I realized it was all four brakes engaging at the same time.

Rotors are straight (okay circular) and within specs. There is no vibration or pulse when applying the brake.

The pedal is hard and not spongy or soft but I honestly don't know if there is any pedal movement b/c if there is I didn't see of notice it.

Pretty much everything has been changed or eliminated except ABS pump (not control module since removing the ABS fuse did not help), brake booster, master cylinder and the hoses.

I will do a thorough test of the master cylinder and booster. Don't know how to check the hoses.

Thanks again

Have you?

Mazda customer service phone numbers and support

Mazda TSB - Information About Mazda Technical Service Bulletins ...

If not, what have you to loose?

I think I would disconnect and plug the vacuum line to the booster then check I can still stop if I want to - probably using both feet on the pedal. Drive it and test it. That will at least eliminate (or maybe expose) the booster as the culprit. My money is on the booster.

Hello

Stop driving like that, but not because wear concerns in the brake pads or discs, but because you're overheating the bearings inside the hub and that could lead to a catastrophic wheel seizure, specially at high speed, believe me I have witnessed it.

If you feel a spongy brake pedal, you probably have air in the system, inspect the brake hoses, as they tend to become porous with age.

You won't see the pores, they're too small; in fact they are so small that won't allow liquid leaks, but will allow air getting into the lines, check for small cracks and hardened rubber, replace as necessary and purge the brake lines.

The same applies to the brake pressure distribution valve; if you see the mounting area is wet, replace it.

Regards

i've had brake hose problems in the past. check that the hoses are'nt balloning while pumping the brakes

Is there a vacuum storage unit on the car? It's just a can connected to manifold vacuum.

Has the vacuum been measured?

If the brakes are being applied, then you really only have the vaccum system and the return spring in the master cylinder.

Try BLOCK the wheels, disconnect the vacuum boost line and plug it, start the car and see how easy it is press the pedal and how fast it returns.

My money is on the ABS system in spite of the fact that you pulled the pump fuse. I suggest you get your hands on an OBD2 tester and interrogate the system. I think it's a bad sensor or controller. Nasty problem to diagnose.

i suggest getting a video camera and an infrared thermometer.

Do another test with passenger videoing (firstly cold discs & drums) then pedal area to show no application of the brakes during test. Drive until it stops as you describe. Video slowing down (i.e. show the outside, & or speedo) and then pedal area again. DO NOT stop the video, - go and use thermometer again to show temp of discs & drums, & record on video.

Write a stat dec as to details & conditions of video & that video is genuine. Email to NHTSA and copy to Mazda Japan. If you don't get a reply in 48, post to youtube.

You have no obligation to advise MazUS for a THIRD time - odds on the problem is known, they are just fobbing you off.

Some fantastic answers here to what is a failed master cylinder.

Tell us how a failed master cylinder applies the brakes.

See post #2. This is the usual fault that causes such problems. Usually all that is needed is to find out why the pedal is not fully returning back up......

I have heard of bad flex lines doing this, but only one wheel at a time, not a pair or all together that wasn't the master cylinder/Booster.....but the problem is known since before boosters were normal....but as the two units are often one unit nowadays, we have to be careful what we say!!!

A good test would be to push a pad back while someone observes the fluid through the topup cap, I don't think that it will get pushed back nor will the fluid level rise....problem found.

I wanted his explanation. He must have a reason for the statement. He probably didn't read #2 anyway.

Guessing, but because it has been written several times already, he maybe did not want to repeat it again....just a thought!!

I think the fox got the chicken.

A swollen cup can also cause these symptoms, blocking off the hole to the reservoir. But not likely.

Most likely (as already said) pedal not returning or adjuster loose & turned by itself. Check this before you lose your car for more days. & money.

Probably you can find the hydraulic circuit online somewhere (Google is your friend) then study it thinking about the symptoms.

I think Andy is correct with the master not returning. There may be an adjustable push rod connecting the brake pedal arm to the master cylinder / booster. Take a look under the dash to see it. Here goes another of Bob's words of wisdom. "If there is no play in the movement of the pedal, the master is not released." Good luck.

Send the expected money to Andy. I am sure he will send me something.

Lyn's words of wisdom, ""If there is no play in the movement of the pedal, get a bigger hammer".

I have not posted a follow up yet b/c I waiting for parts.

FS is "positive" it is the ABS pump b/c they say the lines would not bleed properly until they bypassed it.

I will let them attempt the repair...they say they will back the diagnosis w/ no pay if wrong. We shall see. It will take 3 to 5 days just to receive the part. Maybe the hoses should be replaced at the same time????

Of all the parts not replaced yet that is the most expensive.

More information to come. Thanks everyone.

I have earned a living repairing vehicles from 1967. There are times that we just do not know what is wrong. But the landlord and the electric company all want to get paid.

No one can stay in business spending 8 hours only to find out that there was a poor connection on a fuse. So when there must be a guess, there are two ways to approach it.

1) replace the most expensive and hard to install part(along with as many associated parts with it). Get paid, and if you guessed wrong, eat the cheaper parts.

2) explain to the customer the issue, and replace the cheapest, or easiest part and hope the customer understands your reasoning is for his pocket.

Please understand this is not when a mechanic is just too lazy to diagnose. It is when there are just no answers.

If you are not committed, test the push rod. Good luck. I hope the Fstone people have a chance to read this before you have to pay the bill.

Surely the next replacement must be the master cylinder, a relatively cheap unit that should be seasy to source and replace?

Nice of you to say it but I think Ace was the first, post #2 anyway (from memory), I was just agreeing with him as that is the reason in my experience.

Your quote is fully correct and useful as well, many thanks.

sounds good aa

If the Brake fluid were contaminated with power steering fluid would it act like this? OP didn't mention it but did you add brake fluid for any reason lately? Also many oil change shops top off the fluids when they do an oil change. They could have made a mistake.

No

No.

The first thing that happens when petroleum bassed fluid is added to a master cylinder is the rubber bellow gasket balloons into an oversized piece that will never fit back into place. No mention of this by OP.

Has the master cylinder been separated from the booster recently?

If so, perhaps while they were disconnected the 'finger' protruding from the booster which pushes on the piston in the master cylinder was adjusted in length - it is adjustable - if it has been lengthened - ie screwed out - what you describe will occur.

I made that mistake once years ago and immediately adjusted it back - the problem went away.

GA for good post.

Eminently possible.

Bad flex hoses. There is very little force from the pads to push the fluid back to the master cylinder, but plenty of force from the master cylinder to the caliper. Old hoses swell shut on the inside blocking the fluid. -- JHF

Do you think that both hoses got the exact same internal damage at exactly the same time?

Unlikely, which is exactly why problems of this sort are most often associated with the ABS system.

The single point of failure is the master cylinder, with a little help from the booster sometimes.....there is no other SPF.

Changing anything else first is simply wrong......

The ABS can be turned off on most cars, usually by unplugging a power cable, so that takes care of the ABS in a simple way......I believe someone mentioned it already.....

Remember, an ABS failure is NOT allowed to cause brake problems, other than the loss of the ABS. (most people will not even notice its missing till its wet and slippery and they are driving too fast).

On my first ABS fitted car (many years ago), I had never had the experience of feeling the pedal vibrate in the first 6 months, I thought it wasn't working, so I asked my mechanic, he said just brake hard when its wet.....that proved it worked just fine!!! I'm just a great driver!!!

I believe I am correct when I say that the "standard" reason for this type of failure is something to do with the master cylinder. I am sure the mechanics amongst us will generally agree.....anything else should be tried only when this has been carefully checked out....

I thought ABS stood for Absolute Bull Sh*t. I might not be perfect, but I've avoided accidents for decades. I'm not willing to hand over control to a machine that can't die.

LOL!!!

The German Insurance companies would have agreed with you say 20 years ago, they loaded up cars with ABS with a 27% larger premium. The same for 4WD.....

The reason was simple, statistically then, these cars simply had more accidents!!!

"I can drive like a lunatic as I have ABS and/or 4WD and it will always save me!"

Yesterday we force-paid (similar to force fed) a shop to install a new master cylinder and were able to drive it for about 20 minutes with no problem. Couldn't go any farther due to time constraints and the need to have two vehicles traveling together in case of failure of repair. We aren't 100% sure yet but it may be fixed.

Andy,

you obviously deserve a GA, from me and I would think a few others.

I am not a motoring afficiionado, have n't put paneer on a car since my daughter was given her boy friends car whilst she was at UNI.

The BF and did do some work on it but that was in the dim and distant past - and I have nearly always been satisfied with the local garages.

Good Luck

Sleepy

In terms of garages/workshops there are the good, the bad and the ugly - some use finesse and some are butchers, you have been fortunate, but we have to choose carefully.

A friend is a wrecker and got a relatively new car recently for wrecking. Its glove box was full of receipts for work 'done' - he said if even a small portion of the work charged for had been done the engine would not have died so early and probably left the previous owner with a very poor opinion of the marque ( a Peugeot in this instance) where the problem was not with the vehicle but with the chosen mechanic/workshop because none of the maintenance paid for had been done!!!!

I thank you all for your replies but most of all I thank those of you who would not ease up on insisting it was the Master Cylinder. I had to lie and say my father in law made me get the MC checked b/c he was angry that I had not had it repaired yet and his daughter and grandchildren were at stake. After: FIRESTONE --three times (nothing found then rear caliper or hoses or ABS pump) MAZDA --twice (nothing found then front calipers sticking) PRECISION BRAKES --2 days in shop (didn't get to it first day) didn't know how to check a MC pistonrod. BENTON TRANSMISSION -- 2 days (didn't get to it first day) didn't know how to check a MC pistonrod. A TIRE SHOP took time to listen and do the repair you guys suggested. It just took 8 visits to 5 places to get it repaired. When I told them (each and every one of them) about the responses on CR4 they all reacted smugly and as if they knew better. None of them wanted to listen or to do the MC. They all said bad MC causes soft pedal...period. Kind of scary that I'm probably the rule rather than the exception. THANKS, THANKS, THANKS!!!

Firstly, a big compliment to you for naming names, except you missed out on the addresses....I feel you should add those as soon as possible for other CR4 people in your area......

We are all pleased that it was fixed before anyone was hurt (like your Father-in-law is/was!).

It shows the "quality" of such workers nowadays = CRAP!!!

Don't forget that Ace Boeringa Post #2 was the first one who named the bad part exactly, the rest of us were just agreeing with him!!!

Okay, at the risk of Jeff Foxworthy making a new bit about "Your mechanic might be a dumb@$$ if he can't tell what's wrong with your brakes..." Firestone in in Benton, Arkansas I-30 exit 121.---Three visits, none correct Mazda in Little Rock, Arkansas Colonel Glenn Rd.-----Twice and neither one correct Precision Brakes, Benton, Arkansas Military Road. ---2 days & IDK how 2 check pushrod Benton Transmission, Benton, Arkansas Hwy 5 at Congo Rd.----2 days & pushrod??? Ramsey Tire & Auto Center, Bryant, Arkansas Reynolds Rd.-----Did as asked prob fixed. Thank you Ace and everybody, Goodnight Johnboy...

The good news is that you and your family are safe again. As much time and money as you spent are all small compared to having an accident, and the repairs and expences that would have followed, without considering any personal injuries.

As far as the repair shops not knowing how to check for the issue, BS. All one needs to do is sit on the sill plate of the door opening, and grab the brake pedal with one hand. As it is pushed in the forward direction, there must be a slight increase in pressure required as the master cylinder fights the return spring. If you can not feel the distinct increase in resistance, there is no play. At times there is no play at rest, and, that is acceptable, IF you can pull up on the pedal, and it will move upward. That just means that the weight of the pedal assembly is taking up the play.

But in your case, there was no play either up, or down.